A process is described for preparing .beta.'-sialon comprising heating a mixture of particles comprising from 70% to 90% by weight of particles of an aluminosilicate material and from 30% to 10% by weight of particles of a carbonaceous material at a temperature in the range of from 1300.degree. C. to 1600.degree. C. in nitrogen in a vessel in a manner such that the particles are in substantially continuous motion relative to each other and relative to the nitrogen. The residence time of the particles in the vessel does not exceed 3 hours.